Recently, various optical pickup devices have been developed which can record onto or playback from different optical recording media, depending on the optical recording medium employed. For example, there are devices which record and play back DVD (Digital Versatile Disks, including -ROM, -R, -RW, etc.) and CD (Compact Disk, including -ROM, -R, -RW, etc.) using one optical pickup device. In such optical recording media, visible light having a wavelength of about 650 nm is used with a DVD so as to improve the recording density whereas near-infrared light having a wavelength of about 780 nm is used with a CD, because there are some recording media that have no sensitivity to light in the visible range. Therefore, in an optical pickup device which can be shared by these two types of recording media, a so-called two-wavelength beam scheme is employed wherein light beams of two different wavelengths are used as the irradiation light.
In the case of using a DVD or CD, when the disk thickness and numerical aperture are different between the two optical recording media, the optical pickup device needs to have different focusing actions for light of the different respective wavelengths in order to perform recording/playback. For example, there is an approximate unified standard wherein the disk thickness is 0.6 mm and the numerical aperture NA is 0.6 for DVD recording/playback, and the disk thickness is 1.2 mm and the numerical aperture for CD recording/playback is in the range 0.45≦NA≦0.52.
It is known in the prior art technology to have one surface of the objective lens be an aspheric surface with a ring-shaped diffraction grating that is centered on the optical axis. Such a design enables miniaturization and cost reduction of the optical pickup device to be achieved, as disclosed, for example, in Japanese Laid-Open Patent Applications 2000-81566, 2001-235676, and 2002-109775. In the optical pickup devices disclosed in these patent applications, the variation in spherical aberration due to the thickness of the protective layer is, ideally, canceled by the effect of the diffraction grating so as to form high quality focus spots for the two kinds of optical recording media using a single objective lens.
Also, Japanese Laid-Open Patent Applications 2001-235676 and 2002-109775 disclose technologies wherein a diffraction optical surface that is formed on an aspheric surface is such that the phase difference at a specified position of the aspheric surface is discontinuous, to thereby form a high quality beam spot even for an optical recording medium that requires a small numerical aperture and does not use any of: (a) a liquid crystal shutter, (b) an aperture stop formed of a wavelength-selective filter, etc. or (c) an aperture limiting method which mechanically switches among multiple apertures.
In the objective lens disclosed in the Japanese Laid-Open Patent Application 2000-81566, even when using a small numerical aperture, the light flux passing in an outer region is corrected for aberrations using the same diffraction grating as that for the inner region. Therefore, a high quality beam spot is not produced for both wavelengths used for recording/playback.
Also, the objective lenses disclosed in Japanese Laid-Open Patent Applications 2001-235676 and 2002-109775 are constructed so that the diffraction optical surface is discontinuous at a specified position, and different phase difference functions are employed for the inner region versus the outer region relative to the specified position. However, if the two phase difference functions are such that the phase difference between them is not an integer multiple of 2π at the specified position, the wave-front aberration of the transmitted light flux increases. However, making both phase difference functions such that the phase difference between them at a specified position is an integer multiple of 2π decreases the design freedom in selecting an appropriate diffraction optical surface, which may be a problem.